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                   Time Travel and the Laws of Physics
                   -----------------------------------
                       An Exercise in Speculation
                        by Joey Swails  (c) 1992


    Wouldn't it be wonderful if travel to another time was possible.
Time travel has been a mainstay of fantasy and science fiction for
almost a century, from H.G. Wells to Star Trek. This short essay shall
attempt to discuss the concept in the framework of what is currently
held to be true in the realm of physical science, and how current
theories might apply.
    I'm not a professional physicist, just a well read amatuer with a
couple of underclassman physics courses to my credit. Nothing I have
to say on the subject requires a doctorate in Theoretical Physics to
comprehend. I've always been impressed by the fact that, unlike most
of his collegues, Einstein felt that the average intelligent person
could grasp the ideas of Relativity without the heavy mathematics; to
him it was an indication of the fundimental truth of what he was
working on. To that end, he wrote several highly readable "layman's"
books on the subject himself. I gleaned EVERYTHING in this essay from
other writers, Einstein, Hawking and Sagan among them. And Arthur C.
Clarke, Larry Niven, and Isaac Asimov; these gentlemen, while best
known for their science FICTION writing, are also excellent SCIENCE
writers as well!

And now, from high atop the shoulders of giants...


    Most people have an intuitive grasp of what is meant by "time
travel", at least in a personal sense. One activates a machine or says
the magic words and is "transported" bodily to a different "point in
time", be it the past or future. Sounds simple. But in a universe that
is observed to obey certain "rules of behavior", in practice it may
not be as simple as it sounds.


    Why should we assume that certain laws of physics should apply to
time travel? Aren't new theories of physics often proposed that
supersede older theories, showing them to be incorrect? And therefore,
couldn't even newer theories allow for time travel to be possible?
    The popular misconception is that when a new physical theory is
proposed, it renders preceding one's obsolete. This may have been true
in the case of Copernicus, where what is being superceded is basically
a myth. In modern theoretical physics, the distiction is not so clear
cut. The value of a theory is in it's power to predict the behavior of
the universe by logical inference. Physical theories apply logical
rules to analyzing a specific facet of the "universe of discourse",
that reality which is accessable to everyone, either directly through
the senses, or "second hand" through the use of insturments of
measurement. To invoke logic, the proposed theory need only be
logically self- consistent, even if what it descirbes applies only in
very limited cases.
    When Einstein published his theories of Special and General
Relativity, the planets did not suddenly alter their orbits. We still
use Newtonian laws of motion to predict the trajectories of
spacecraft, and they appear to work quite well WITHIN THEIR LIMITS.
What happens is that the limits are expanded, and new theories are
proposed to describe behavior outside the old law's limits. The old
laws still hold; new laws usually apply only to new areas of
observation. They build on, rather than eliminate, the old laws.

    So, let's speculate on some of the concepts of time travel. For
the purposes of speculation, we must make some assumptions and see
where they take us. For logic to work, the only requirement is that
the assumptions are internally consistent, i.e. if something is true
in case A(1) it is also true in case A(2) - no changing the rules as
you go along. The overall assumption I'm making is that certain laws
of the universe will always hold, and time travel can only be possible
if NONE of them are violated in the process. Specifically, I refer to
the Conservation of Matter and Energy; the Conservation of Motion;
Entropy; and Causality. Perhaps I'm nitpicking, but not really - we'll
leave Relativity and Quantum Mechanics aside for now and concentrate
on predictive theories that have meaning across a broad spectrum of
universal models, from Newton to Hawking.
    Also because Relativity tends to knock out time travel in the
first round, which wouldn't be much fun; and Quantum Mechanics STILL
can't find that damn tachyon particle (predicted to travel "backwards
in time"), and at the quantum level, we can't even tell which way even
NORMAL time is "moving" anyway...

    At least, not yet...

The Basic Paradox of Time Travel
--------------------------------
    When anyone speculates about time travel, one of the first things
encountered is known as the Grandfather Paradox, the origination of
which is credited to Einstein. The basic idea is this:
    I invent a time machine. Since I always hated my nasty old
grandfather, I travel back in time to when he was a young boy and
murder him. The perfect crime!
    But I've gone and killed him before he met my grandmother, so
therefore I was never born, and of course couldn't have invented a
time machine.
    So I couldn't have killed him. So he sires my father, who sires
me, who invents a time machine...
    Me and my time machine must both exist and not exist. There lies
the paradox. Any process that interferes with the past in a
self-cancelling way is broadly called a Grandfather Paradox.
    This is the big one: Causality. Our belief in ANY physical laws at
all presupposes cause-and-effect; if this, then that. Maybe you don't
WANT to kill your grandfather, but the possibility would exist that
you COULD, and that's all that matters. The effect is made to come
before the cause, and causes the cause not to effect the effect.
Fundimentally illogical, and was reason enough for Einstein to consider
time travel impossible.
    But it gets even stickier when we consider along with it the next
law we will speculate about - Conservation of Matter and Energy.

Conservation Laws and Time Travel
---------------------------------
    The law basically states that matter and energy can be converted
into one another, but can NEVER be destroyed (as in totally
eliminated) from the universe. Conversely, neither one can be created
spontaneously out of nothing. A time machine that travels from now to
1000 years ago, can be said by any test available, to have appeared
out of nothing, and can be said to also have dissappeared INTO nothing
(from it's original starting point). This also goes for any energy
that made the trip with it. As long as it remains translated out of
it's original point in time, there's extra matter running around. But
it doesn't stop there.
    Let's assume that I build a time machine in my workshop. Then I
travel back to the past, visit the workshop, and remove or destroy
some irreplacable part of the machine. How then would I be able to
build the machine? Paradox again!
    Let's continue further along this line. I travel back in time to
1pm yesterday and remain there for one hour, then return. Upon
returning, I again travel back to 1pm yesterday. I meet myself! BIG
paradox!
    Some may want to believe in some "grand overall" theory of
Conservation. It goes something like this - since the time machine
disappeared in one time period, and reappeared in another, there is
still only one time machine in any one place at a time, so some kind
of "overall conservation" from the beginning of time to infinity, is
preserved. There's no evidence that this should be the case, but no
clear evidence that it could NOT be, so let's be gracious. Now, a
problem arises when I time travel from various points in the future to
the SAME point in the past. If I assume I could go on doing this
indefinitely, a HUGE pile of doppelgangers begins to accumulate at a
single point in time, seemingly replicating me and my time machine ad
infinitum. Clearly a violation of logic as well as conservation.
    For MORE fun, let's say we all agree to return to the same point
in future space/time. Now, how do we all occupy the same point in
space/time?

    Now I here someone in the last row say, "OK, perhaps the process
won't work if you try to go somewhere (or is it someWHEN?) that the
time machine already is." Then the question arises, what constitutes
"the time machine"? The collection of atoms and energy packets that
make up it's physical structure? OK. But all of those components
existed in SOME form before I built a time machine out of them. Why
would matter behave differently once it was part of a time machine?
Does it mean I can't travel back to my workshop to the time before I
assembled the machine, when all I had were component parts? Or I can't
travel to the time at the foundry where/when they cast the steel for
the outer hull? A particle of matter or energy has no "knowledge" of
what it is a "part of" at a given point of space/time (at least, an
isolated particle has never been observed to behave differently due to
where it "came from".) There's no clear place to "draw the line" -
either it's drawn with no exceptions (paradox) or it's not drawn at
all (impossibility)!

    That leaves the Law of Conservation of Motion. Newton described it
first, but Einstein was the first to offer a viable explanation for
it. (I know, I promised "no Relativity", but this will be over in a
second.) A rigid fourth dimensional continuum is required for his
explaination to work; and it DOES work - If E=mc^2 was incorrect,
thermonuclear bombs would not explode. I think we can all agree that
they DO. For anything to move around in the time dimension, it must
move faster than light, acquiring infinite mass and energy along the
way. Once you EXCEED the speed of light, you're traveling backwards in
time - but GETTING THERE could be hazardous to your health.
    Physical time travel clearly violates any law of motion, as motion
ALWAYS relates to time. This effects conservation of motion, rules of
kinetic energy, even the law of gravity.(At least, any law of gravity
I can think of!)

Entropy and Time Travel
-----------------------
    Finally, there is entropy (sounds like a philosophical statement,
doesn't it?) - the tendency of matter and energy to move from order to
disorder, embodied by the Second Law of Thermodynamics. This tendency
toward randomization gives us the most obvious impression of "moving
forward in time". It's not easy to observe the decaying orbit of a
planet, or the breakdown of a lambda particle, but try to get an
exploded bomb back into it's casing. That's entropy. It's happening
all the time, to everything in the universe, and it's rate of decay
for a given type of particle is steady in the extreme. This makes
things like atomic clocks and Carbon-14 dating possible.
    There's also the products of this breakdown to consider. A human
body going through it's biological processes is a walking bundle of
changing energy states, some changing to higher states, but more
changing to lower states - always a net loss. Entropy again.
    So say I travel to the past, and while there I'm breathing the air
of the past, breaking down the bonds of the O2 molocules and binding
them to carbon, deriving energy for my metabolism from the process. So
I'm exhaling particles with lower energy states than they had when I
sucked them into my lungs (or less of them; a moot point, because
matter and energy are the same thing ultimately.) When I return to my
original time, how do I recover all of the exhaled particles that now
have an "incorrect" energy state relative to all the other particles
in their "new" time-frame? I'd HAVE to, in order to perserve any idea
of universal entropy; if I leave them "behind" they stay there until
the end of infinity, and we never "balance the books" of the total
energy state of the Universe.

Quantum Mechanics and Time Travel
---------------------------------
    I suppose we're going to HAVE to bring up tachyons. I know, it IS
Quantum Mechanics, but the tachyon is the darling of time-travel
enthusiasts, so in anticipation of it being brought up by SOMEBODY,
let's discuss it a bit.
    Tachyons, though not yet detected, are predicted to exist by
Quantum Theory. Let me dig out the physics text - what physicists are
looking for is the decay of a lambda particle into a proton plus a pi
meson. They predict they will find a nonzero value for the beta
component of the spin of the proton (whew!), implying the release of a
particle with reversed spin and charge - meaning it's moving "backward
in time"!
    The problem is they have not found such a reaction to take place.
Particle accelerators have been blasting away at the particles in
their cloud chambers for years now, and the shy tachyon still refuses
to "show itself". Most scientists studying the subject now believe
they will find something wrong with the model that predicts the
existence of tachyons.

    But let's assume that they DO exist. Can a machine (or a human
body) "make like a tachyon" and boogie backward in time? All you have
to do is start moving backwards and wait for the past to catch up with
you.
    The theory that predicts tachyons says that in order to do that
trick, the mass in question would have to instantly reverse the spin
and charge of every sub-atomic particle in it. But since the mass is
NOT made of tachyons in the first place, to impose this on any other
kind of particle will have the distressing side-effect of converting
the entire mass into anti-matter. MAJOR fireworks...

    This points out the error in the logic of "if tachyons can move
backwards in time, then why can't something ELSE do it too?". If you
subscribe to the theory that allows for the existence of the backwards
moving tachyon, you can't selectivly ignore any of the corrolaries
that go along with the prediction. Tachyons are a LIMITED SPECIAL
CASE, and QM is full of them. Sauce for the tachyon is not necessarily
sauce for the proton, in a manner of speaking. It's this plethora of
limited special cases that makes coming up with a Grand Unified Theory
to cover ALL the cases so difficult to formulate.

Relativity and Time Travel
--------------------------
    Before anybody gets upset, let me point out that the Relativity
Laws provide the ONLY available means to travel in time that we know
about. It's called time dilation, and it occurs during acceleration of
mass. It's almost undetectable at speeds we normally deal with, but
experiments with atomic clocks in space capsules have borne out the
theory - it happens. An accelerating mass experiences the flow of time
at a slower rate than a mass at rest. Travel to Proxima Centauri, a
trip of 4.3 light years, and accelerate steadily to 9/10ths of
lightspeed till you're halfway there, then de-accelerate down to rest;
now turn around and go back. To you, the round trip took about 25
years. Returning to earth, you find that about 3000 years have elapsed
while you were gone! The closer you approach lightspeed, the more
drastic the effect. A one-way trip, and only to the future, but it
works!
    Also, the mathematics of Relativity predict that some strange
things may occur to a mass that manages to travel at a trajectory that
passes the outer event horizon of a rotating black hole (look up
Penrose diagrams in a text on Relativity for more about this effect.)
Suffice it to say that it MAY allow a mass to travel outside it's
normal space-time axis. The theory can't predict exactly HOW this
effect, if it exists, would manifest itself; it could mean
superluminal motion, it could mean time-travel, or it could mean
emerging into a different universe entirely. It might also not follow
the prediction at all (remember tachyons? Theory says we should find
them, but we haven't.)
    Plus there are the practical problems. Converting enough energy
for a controlled fly-by of a black hole would require a LOT of logs to
throw on the fire, to say the least! Something like the output of your
average star should suffice. And then there's the question of what to
build the hull out of - something that can handle the hellish
radiation spewing out of the event horizon, as well as the
acceleration, gravity pull and tidal forces involved. A few gigatons
of neutron star material should do quite nicely, I would think. And
also the problem of controlling the trajectory that you take, which is
what determines where/when you will end up, calculated to many
fiendish decimal places. The tiniest of mistakes and you end up being
INTIMATELY aquainted with a black hole...

    It may be fun to think about, but it's not likely that this
"loophole" in Relativity will lead to a practical time machine!

The Letter of the Law
---------------------
    I've assumed throughout all my speculations here that the physical
laws involved cannot be broken. To be fair, I've accepted that there
can even be some slight bending in the cases of Relativity and Quantum
Mechanics, which really means only that our understanding of them is
incomplete. I've been really stubborn, however, regarding Conservation
Laws. Is this justifiable? I believe it is.
    One of Einstien's postulations was that the universe is the same
everywhere. The rules governing the behavior of mass and energy are
the same no matter where you are. And in ALL known cases, the Laws of
Conservation hold rigorously, from the behavior of galaxies to the
behavior of particles. They are what Quantum scientists use to find
new quantum states. The existence of the neutrino was postulated using
them, and this very elusive particle has been successfully detected,
behaveing exactly as the Laws say it should.
     If we allow exceptions, we have some serious problems trying to
find another way to explain all the phenomena occuring in our
universe. Conservation is a FACT, as cast in immutable stone as
anything like it can possibly be.
    Any theory is an exercise in speculation, and must start with
assumptions. I chose the ones that allowed for the reality of ghost
particles and hydrogen bombs. I think I'm in good company. But what
truly matters is self - consistancy. Make you own assumptions and see
where they take you. However, once you make them, stick to them, or
you can't expect truth to emerge.

A Few Direct Quotes
-------------------
    First, from the fictional character of Lazarus Long, the oldest
living human, (when last seen, over 2000 years old!)created by Robert
Heinlein:
    "What are the facts? Again and again and again - what are the
FACTS? Shun wishful thinking, ignore divine revelation, forget what
"the stars foretell", avoid opinion (especially your own), care not
what the neighbors think, never mind the "unguessable verdict of
history" - what are the facts, and to how many decimal places? You
pilot always into an unknowable future, and facts are your single
clue. GET THE FACTS!"

    Lastly, from Isaac Asimov, a NON-fictional character (but quite a
character, nonetheless):
    "It is not required of The Laws of the Universe that they manifest
themselves in a way that is convienient."

    In other words, "anything is possible" might simply be impossible.
The universal dice just might be loaded in such a way that things like
time travel and faster-than-light speeds are TRULY IMPOSSIBLE, now and
forever, no matter how much we may wish it were otherwise.

    But take heart. There are enough wonderous, mind-boggling things
left in the Universe to keep us entertained for a very long time to
come...

    ...but please, no time machines.